1. Field of the Invention
The invention relates to reduction of nitric oxide content in combusted gas streams wherein the temperature of the combusted gas streams vary over a wide range. Such conditions are found, for example, in diesel and in other hydrocarbon-burning engines.
2. Prior Art
The prior art teaches exhaust gas nitric oxide reduction through reaction with ammonia over a nitric oxide reduction catalyst at temperatures above about 260.degree. C. The prior art also teaches that ammonium halides and especially ammonium bromide on activated charcoal catalyze the reduction of nitric oxide with ammonia at temperatures in the range from 100.degree. C. to 170.degree. C. This latter system has however not been used in practical applications such as engine exhaust streams wherein the temperature of the gas contacting the catalyst will exceed about 170.degree. C. because the catalyst will be destroyed if subjected to exhaust temperatures significantly above about 170.degree. C. The more thermally stable nitric oxide reduction catalysts as are used on engine exhausts are, however, of reduced efficiency below about 260.degree. C., and are relatively inefficient below about 220.degree. C. Accordingly, if one makes use of only the normal prior art high temperature stable nitric oxide reduction catalysts on exhaust gases, high nitric oxide contents occur whenever the temperature of the exhaust stream being fed into the catalyst bed is below about 220.degree. C., and relatively high nitric oxide contents tend to occur between about 220.degree. C. up to about 280.degree. C. This often occurs in practice. For example, at idle a diesel engine may have an exhaust temperature of only about 100.degree. C. This is clearly a highly undesirable state of affairs.